Wood naturally contains moisture. However, for wood lumber to be useful for building, the moisture content must fall within certain constraints. Too much or too little water in the lumber can cause serious problems in the subsequent processing and use of the lumber, including warping or difficulties with painting or gluing. In order to provide a useful product, most lumber mills dry their lumber under controlled processes, such as in a kiln, prior to sale. It is difficult, however, to ensure that every board in a given charge will have exactly the same moisture content after wood drying kilns.